A previously unknown species of parasite has been discovered in the livers of Pacific saury sold in supermarkets. According to an international research team led by Nippon Veterinary and Life Science University, the parasite is a protozoan belonging to the genus Goussia. It is a single-celled eukaryotic organism with a spherical shape measuring about 20 micrometers (0.02 millimeters) in diameter. The researchers believe it poses no risk to human health.
The discovery stemmed from a seminar led by Associate Professor Toshihiro Tokiwa, a parasitologist in the Department of Veterinary Medicine at Nippon Veterinary and Life Science University. While examining parasites in Pacific saury purchased from supermarkets, he came across an unfamiliar organism. Further investigation revealed that it was a previously undescribed protozoan. Protozoa are single-celled eukaryotic organisms. Well-known examples include the malaria parasite, which causes malaria, and Toxoplasma, which can be transmitted through cats.
Genetic analyses showed that the newly discovered protozoan belongs to the genus Goussia. Members of the genus Goussia, commonly known as coccidia, parasitize a wide range of hosts, including amphibians, freshwater fish, and marine fish. The newly discovered species parasitizes marine fish.
Further analyses revealed that Goussia does not form a single evolutionary lineage descended from a common ancestor. Instead, the genus groups together several lineages with different evolutionary origins. In other words, organisms classified within the same genus do not necessarily share the same evolutionary history.
Because parasites rarely leave fossil records, determining when they first emerged is difficult. To estimate when the protozoan diverged into its present-day lineages, the research team used clues such as the evolutionary history of its hosts and the timing of continental separation. The analyses suggested that it diverged from Goussia species that parasitize freshwater fish around 200 million years ago and has since followed its own evolutionary trajectory.
Provided by Associate Professor Toshihiro Tokiwa
Pacific sauries purchased from multiple supermarkets were examined, and the protozoan was detected in 19 of the 21 fish. Last year saw a bumper Pacific saury catch, and many fish had accumulated large amounts of fat, resulting in what are commonly referred to as "fatty livers." Tokiwa recalled that distinguishing the protozoan from fatty liver tissue was difficult because fat droplets in the liver closely resemble the parasite in appearance.
The researchers found that protozoa of the genus Goussia parasitize only the liver of Pacific saury, and that transmission between fish is believed to occur via feces. In addition, the infection is thought to occur only among fish, and no cases of human infection have been reported.
The team plans to investigate the effects of Goussia parasitism on Pacific saury and to examine infection rates in other fish species, including sardines, horse mackerel, and mackerel. Because previous studies have reported seasonal variations in protozoan infection rates in other fish species, the researchers also hope to clarify the dynamics of infection.
"Coccidia have a remarkably beautiful round appearance," Tokiwa said. "Even familiar fish such as Pacific saury still harbor tiny parasites that have yet to be discovered. I have heard that fish parasites are increasingly being used in inquiry-based learning programs for junior high and high school students. I hope students will take the opportunity to discover the small surprises hidden within the living things around them."
The research was conducted jointly by Nippon Veterinary and Life Science University and National Chung Hsing University in Taiwan. The findings were published online in the August 12 issue of the American journal Journal of Eukaryotic Microbiology. Nippon Veterinary and Life Science University announced the results on August 13.
Original article was provided by the Science Portal and has been translated by Science Japan.